// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef CONTENT_BROWSER_HISTOGRAM_SYNCHRONIZER_H_
#define CONTENT_BROWSER_HISTOGRAM_SYNCHRONIZER_H_

#include <string>
#include <vector>

#include "base/callback.h"
#include "base/macros.h"
#include "base/memory/singleton.h"
#include "base/synchronization/lock.h"
#include "base/task_runner.h"
#include "base/time/time.h"
#include "content/browser/histogram_subscriber.h"

namespace content {

// This class maintains state that is used to upload histogram data from the
// various child processes, into the browser process. Such transactions are
// usually instigated by the browser. In general, a child process will respond
// by gathering snapshots of all internal histograms, calculating what has
// changed since its last upload, and transmitting a pickled collection of
// deltas.
//
// There are actually two modes of update request.  One is synchronous (and
// blocks the UI thread, waiting to populate an about:histograms tab) and the
// other is asynchronous, and used by the metrics services in preparation for a
// log upload.
//
// To assure that all the processes have responded, a counter is maintained to
// indicate the number of pending (not yet responsive) processes. To avoid
// confusion about a response (i.e., is the process responding to a current
// request for an update, or to an old request for an update) we tag each group
// of requests with a sequence number. When an update arrives we can ignore it
// (relative to the counter) if it does not relate to a current outstanding
// sequence number.
//
// There is one final mode of use, where a renderer spontaneously decides to
// transmit a collection of histogram data.  This is designed for use when the
// renderer is terminating.  Unfortunately, renders may be terminated without
// warning, and the best we can do is periodically acquire data from a tab, such
// as when a page load has completed.  In this mode, the renderer uses a
// reserved sequence number, different from any sequence number that might be
// specified by a browser request.  Since this sequence number can't match an
// outstanding sequence number, the pickled data is accepted into the browser,
// but there is no impact on the counters.

class HistogramSynchronizer : public HistogramSubscriber {
public:
    enum ProcessHistogramRequester {
        UNKNOWN,
        ASYNC_HISTOGRAMS,
    };

    // Return pointer to the singleton instance for the current process, or NULL
    // if none.
    static HistogramSynchronizer* GetInstance();

    // Contact all processes, and get them to upload to the browser any/all
    // changes to histograms. This method is called from about:histograms.
    static void FetchHistograms();

    // Contact all child processes, and get them to upload to the browser any/all
    // changes to histograms.  When all changes have been acquired, or when the
    // wait time expires (whichever is sooner), post the callback to the specified
    // TaskRunner. Note the callback is posted exactly once.
    static void FetchHistogramsAsynchronously(
        scoped_refptr<base::TaskRunner> task_runner,
        const base::Closure& callback,
        base::TimeDelta wait_time);

private:
    friend struct base::DefaultSingletonTraits<HistogramSynchronizer>;

    class RequestContext;

    HistogramSynchronizer();
    ~HistogramSynchronizer() override;

    // Establish a new sequence number, and use it to notify all processes
    // (renderers, plugins, GPU, etc) of the need to supply, to the browser,
    // any/all changes to their histograms. |wait_time| specifies the amount of
    // time to wait before cancelling the requests for non-responsive processes.
    void RegisterAndNotifyAllProcesses(ProcessHistogramRequester requester,
        base::TimeDelta wait_time);

    // -------------------------------------------------------
    // HistogramSubscriber methods for browser child processes
    // -------------------------------------------------------

    // Update the number of pending processes for the given |sequence_number|.
    // This is called on UI thread.
    void OnPendingProcesses(int sequence_number,
        int pending_processes,
        bool end) override;

    // Send histogram_data back to caller and also record that we are waiting
    // for one less histogram data from child process for the given sequence
    // number. This method is accessible on UI thread.
    void OnHistogramDataCollected(
        int sequence_number,
        const std::vector<std::string>& pickled_histograms) override;

    // Set the |callback_task_runner_| and |callback_| members. If these members
    // already had values, then as a side effect, post the old |callback_| to the
    // old |callback_task_runner_|. This side effect should not generally happen,
    // but is in place to assure correctness (that any tasks that were set, are
    // eventually called, and never merely discarded).
    void SetTaskRunnerAndCallback(scoped_refptr<base::TaskRunner> task_runner,
        const base::Closure& callback);

    void ForceHistogramSynchronizationDoneCallback(int sequence_number);

    // Internal helper function, to post task, and record callback stats.
    void InternalPostTask(scoped_refptr<base::TaskRunner> task_runner,
        const base::Closure& callback);

    // Gets a new sequence number to be sent to processes from browser process.
    int GetNextAvailableSequenceNumber(ProcessHistogramRequester requester);

    // This lock_ protects access to all members.
    base::Lock lock_;

    // When a request is made to asynchronously update the histograms, we store
    // the task and TaskRunner we use to post a completion notification in
    // |callback_| and |callback_task_runner_|.
    base::Closure callback_;
    scoped_refptr<base::TaskRunner> callback_task_runner_;

    // We don't track the actual processes that are contacted for an update, only
    // the count of the number of processes, and we can sometimes time-out and
    // give up on a "slow to respond" process.  We use a sequence_number to be
    // sure a response from a process is associated with the current round of
    // requests (and not merely a VERY belated prior response).
    // All sequence numbers used are non-negative.
    // last_used_sequence_number_ is the most recently used number (used to avoid
    // reuse for a long time).
    int last_used_sequence_number_;

    // The sequence number used by the most recent asynchronous update request to
    // contact all processes.
    int async_sequence_number_;

    DISALLOW_COPY_AND_ASSIGN(HistogramSynchronizer);
};

} // namespace content

#endif // CONTENT_BROWSER_HISTOGRAM_SYNCHRONIZER_H_
